Halogen-bonded co-crystal containing 1,3-diiodoperchlorobenzene and the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane resulting in a zigzag topology

The formation of a co-crystal with a zigzag topology sustained by halogen bonds involving the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane acting as a two connecting node is reported.


Chemical context
A continued focus for crystal engineers is the formation of molecular networks held together by non-covalent interactions (Vantomme & Meijer, 2019). Still today, research on these purely organic materials continues to lag behind related areas such as metal-organic frameworks as well as supramolecular coordinated solids. Co-crystallization has proven to be a successful approach in the formation of these extended organic solids (Gunawardana & Aakerö y, 2018). As in all types of network design, the components of these co-crystals must be carefully considered to ensure complementary supramolecular donor and acceptor sites that will allow for self-assembly of the multi-component solid. A highly utilized and well-established non-covalent interaction is halogen bonding, which is defined as the interaction between an electrophilic region on a halogen atom and a nucleophilic region on a different atom (Gilday et al., 2015). Overall, the strength and directionality of halogen bonds makes them an ideal supramolecular interaction, along with hydrogen bonds, as a driving force in the formation of co-crystals (Corpinot & Bučar, 2019).
A continued area of focus between our research groups has been in the design and formation of halogen-bonded molecular networks (Dunning et al., 2021(Dunning et al., , 2022Oburn et al., 2020;Sinnwell et al., 2020) that contain cyclobutane-based nodes generated from the [2 + 2] cycloaddition reaction between alkenes (Kole & Mir, 2022;Gan et al., 2018). Recently, we reported the ability to form a molecular salt with a square network topology based upon the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane and the sulfate anion (Santana et al., 2021b). The rtct-isomer, which is the more stable thermodynamic product, is not directly observed from the solid-state [2 + 2] cycloaddition reaction, but rather forms after the photoreaction and an acid-catalysed isomerization reaction (Hill et al., 2012;Peedikakkal et al., 2010).
Herein, we report the solid-state crystal structure of a cocrystal held together by IÁ Á ÁN halogen bonds that has a zigzag topology. In particular, the solid is based upon 1,3-diiodoperchlorobenzene (C 6 I 2 Cl 4 ) acting as the halogen-bond donor while the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (TPCB) behaves as the acceptor. Unexpectedly, the TPCB molecule is found to accept only two IÁ Á ÁN halogen bonds, between neighbouring 4-pyridyl rings, which makes the photoproduct act as a bent two-connected node rather than a four-connected node as seen in the square network topology with the sulfate anion (Santana et al., 2021b).

Structural commentary
Crystallographic analysis revealed that the components of (C 6 I 2 Cl 4 )Á(TPCB) crystallize in the centrosymmetric monoclinic space group P2 1 /c. The asymmetric unit contains a full molecule of both C 6 I 2 Cl 4 and TPCB (Fig. 1) although the crystals formed from a 2:1 solution of the two components. Notably, the TPCB molecule has an rtct-geometry, as expected since we first subjected the rctt-TPCB to an acid-catalysed isomerization. As a result of the isomerization reaction, the bond angles between neighbouring 4-pyridyl rings within TPCB are nearly perpendicular, with all four angles slightly obtuse at 93.64 (7), 96.05 (7), 96.37 (7) and 100.50 (7) . These bond angles were measured from the centroids of the cyclo-butane and the pyridine rings. As expected, the halogen-bond donor C 6 I 2 Cl 4 forms two crystallographically unique IÁ Á ÁN halogen bonds with TPCB. The halogen-bond distances between I1Á Á ÁN4 and I2Á Á ÁN3 i have values of 2.757 (4) and 2.909 (4) Å along with bond angles for C1-I1Á Á ÁN4 and C3-I2Á Á ÁN3 i of 176.58 (15) and 172.73 (16) , respectively [symmetry code: (i) 1 + x, 3 2 À y, À 1 2 + z]. Surprisingly, within (C 6 I 2 Cl 4 )Á(TPCB) only two adjacent 4-pyridyl rings are accepting these IÁ Á ÁN halogen bonds. As a consequence of the observed formula and the lower than expected number of halogen bonds, TPCB behaves as a bent two-connecting node, resulting in a zigzag topology (Fig. 2). The pitch distance observed within (C 6 I 2 Cl 4 )Á(TPCB) is 20.51 (2) Å measured from the centroids of two nearest cyclobutane rings within the chain. Even though the I atoms on C 6 I 2 Cl 4 are found in the meta positions, rather than the para position, this halogenbond donor acts as a nearly linear linker within (C 6 I 2 Cl 4 )Á(TPCB) (Fig. 2).

Supramolecular features
In addition to halogen bonding within (C 6 I 2 Cl 4 )Á(TPCB), the photoproduct TPCB is found to engage in a C-HÁ Á ÁN hydrogen bond, resulting in a mono-periodic zigzag chain (Fig. 3). In particular, this C-HÁ Á ÁN hydrogen bond has a CÁ Á ÁN separation of 3.442 (7) Å and a C-HÁ Á ÁN angle of 148 . It is important to note that the hydrogen-bond-accepting N atom does not accept halogen bonds. The donor H atom for this C-HÁ Á ÁN hydrogen bond is in the 3-position on a pyridine ring that accepts a halogen bond.

Figure 1
The labelled asymmetric unit of (C 6 I 2 Cl 4 )Á(TPCB). Displacement ellipsoids are drawn at the 50% probability level for non-hydrogen atoms while hydrogen atoms are shown as spheres of arbitrary size. X-ray crystal structure of (C 6 I 2 Cl 4 )Á(TPCB) illustrating the zigzag network held together by IÁ Á ÁN halogen bonds. Halogen bonds are represented by yellow dashed lines.

Figure 3
X-ray crystal structure of (C 6 I 2 Cl 4 )Á(TPCB) illustrating the C-HÁ Á ÁN hydrogen bonds between photoproducts forming a zigzag chain. Hydrogen bonds are represented by yellow dashed lines.
These different types of non-covalent interactions were also investigated and visualized by a Hirshfeld surface analysis (Spackman et al., 2021) mapped over d norm (Fig. 4). The darkest red spots on the Hirshfeld surface represent IÁ Á ÁN halogen bonds while the faint red spots indicate the C-HÁ Á ÁN interactions. The adjacent halogen bond accepting 4-pyridyl groups within TPCB generates the two-connecting node and the bent geometry required for a zigzag topology.

Database survey
A search of the Cambridge Crystallographic Database, Version 2022.3.0 Build 364, (Groom et al., 2016) using Conquest (Bruno et al., 2002) for structures containing tetrakis(pyridin-4-yl)cyclobutane in which one pyridyl N atom is within the van der Waals radius of a halogen atom revealed a total of four structures. Two of these structures correspond to the rtct-isomer. One of these, refcode RULHAK, is our earlier report of the tetrahedral network formed between 1,4-diiodoperchlorobenzene and rctt-TPCB (Oburn et al., 2020), in which all four pyridyl N atoms are halogen-bond acceptors. In the other structure, refcode EKUJOM (Santana et al., 2021a), a chlorine atom ortho to a hydrogen-bonded phenol has a geometry-enforced close contact to the N atom.

Synthesis and crystallization
Materials and general methods The solvents such as reagent grade ethanol, dimethyl sulfoxide, chloroform, and toluene were all purchased from Sigma-Aldrich Chemical (St. Louis, MO, USA) and used as received. In addition, resorcinol (res), trans-1,2-bis(pyridin-4-yl)ethylene (BPE), concentrated sulfuric acid, and sodium hydroxide pellets were also purchased from Sigma-Aldrich and were used without additional purification. The [2 + 2] cycloaddition reaction was conducted in an ACE Glass photochemistry cabinet using UVradiation from a 450 W medium-pressure mercury lamp. The occurrence of both the [2 + 2] cycloaddition reaction along with the acid-catalysed isomerization reaction were confirmed by using 1 H Nuclear Magnetic Resonance Spectroscopy on a Bruker Avance 400 MHz spectrometer with dimethyl sulfoxide (DMSO-d 6 ) as the solvent. The halogen-bond donor 1,3-diiodoperchlorobenzene (C 6 I 2 Cl 4 ) was synthesized utilizing a previously published method (Reddy et al., 2006).
Synthesis and crystallization The formation of the halogenbond acceptor rtct-tetrakis(pyridin-4-yl)cyclobutane (TPCB) was achieved by using a previously published approach (Santana et al., 2021a). In particular, the co-crystal 2(res)Á2(BPE) undergoes a [2 + 2] cycloaddition reaction to yield rctt-tetrakis(pyridin-4-yl)cyclobutane as previously reported (MacGillivray et al., 2000). The rctt-photoproduct was removed from the template by means of a base extraction with 0.2 M sodium hydroxide solution along with chloroform as the solvent. The conversion from rctt-to the rtct-isomer was achieved by heating 100 mg of the rctt-photoproduct in a 10 mL beaker with 2.0 mL of dimethyl sulfoxide along with two drops of sulfuric acid. The resulting solution was heated on a hot plate for one hour at 373 K (Peedikakkal et al., 2010). The complete upfield shift of the cyclobutane in the 1 H NMR spectra from 4.86 ppm for the rctt-isomer to 3.86 ppm for the rtct-isomer confirms the quantitative yield for this isomerization reaction. The separation of the photoproduct from the sulfate salt was achieved by a base extraction with 0.2 M sodium hydroxide and again chloroform in three 10.0 mL aliquots. Removal of the chloroform yielded pure TPCB (Fig. 1 in the supporting information).
The formation of (C 6 I 2 Cl 4 )Á(TPCB) was achieved by dissolving 64.0 mg of C 6 I 2 Cl 4 in 2.0 mL of toluene and then combined with a 2.0 mL ethanol solution containing 25.0 mg of TPCB (2:1 molar equivalent). Within three days, single crystals suitable for X-ray diffraction were formed upon loss of some of the solvent by slow evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Intensity data were corrected for Lorentz, polarization, and background effects using CrysAlis PRO (Rigaku OD, 2021). A numerical absorption correction was applied based on a Gaussian integration over a multifaceted crystal and followed by a semi-empirical correction for absorption applied using the program SCALE3 ABSPACK. The program SHELXT (Sheldrick, 2015a) was used for the initial structure solution while SHELXL (Sheldrick, 2015b) for the refinement of the structure. Both programs were utilized within the OLEX2 software (Dolomanov et al., 2009). Hydrogen atoms bound to carbon atoms were located in the difference-Fourier map and were geometrically constrained using the appropriate AFIX commands. Hirshfeld surface of (C 6 I 2 Cl 4 )Á(TPCB) mapped over d norm illustrating the IÁ Á ÁN halogen bonds (darkest red spots) and the C-HÁ Á ÁN hydrogen bonds (faint red spots).

Funding information
RHG gratefully acknowledges financial support from Webster University in the form of various Faculty Research Grants.